Reaction Mechanism of the Sheet Galvanizing

Abstract

The relation between the changes of the alloy layer structures and diffusion paths, for galvanized steel sheets, has been investigated and kinetics of the alloy layer growth and iron dissolution are discussed.
Any type of the alloy layer structures corresponds to one of the diffusion paths on the Fe-Zn-Al ternary phase diagram. The growth of the inhibited alloy layer follows the reciprocal logarithmic low. The outburst structure, where the δ layer is destroyed and penetrated by liquid zinc, grows linearly with time. The growth of the alloy layer containing the thick compact δ layer follows the parabolic law. The inhited alloy layer transforms to the outburst structure by an autocatalytic reaction. When immersion time is short, the alloy layer is considered to be in a metastable state and the solubility of iron at the solid-liquid interface presumably rises above the equilibrium solubility. Therefore, the rate of iron dissolution increases and iron can dissolve in the iron saturated bath. When the alloy layer contains the outburst structure, the δ particles spall off into the bath, which constitutes the mechanism of iron transfer to the bath. Except for the above cases, iron dissolves in proportion to the concentration difference between the equilibrium solubility and iron in the bath.